1. Field of the Invention
The present invention relates to a light emitting diode driving circuit for use in a photoelectric switch apparatus or the like and, more particularly, to a light emitting diode driving circuit detecting the optical output power of a light emitting diode and using the detected output for regulating the clamping voltage of a clamping circuit, thereby automatically controlling driving power of the light emitting diode.
2. Description of the Related Art
In a photoelectric switch apparatus for detecting a transported object on a conveyer belt, it is conventionally arranged such that plural sets of light emitting diodes and photodiodes are arranged along the conveyer belt at intervals of a suitable distance, the light emitting diodes are caused to emit light intermittently at the timing in agreement with the timing of the transportation of the object, and the reflected light from the transported object is detected by the respective corresponding photodiodes of the light emitting diodes, and it is thereby possible to detect presence or absence of the transported object. At this time, while the light emitting diode is intermittently driven by a driving pulse with a rectangular waveform generated by a timing circuit, the driving pulse is arranged to be supplied to the light emitting diode through a light emitting diode driving circuit.
FIG. 9 is a block diagram showing an example of a conventional photoelectric switch apparatus.
Referring to FIG. 9, reference numeral 50 denotes a photodiode, 51 denotes a preamplifier, 52 denotes a low-pass filter, 53 denotes an amplifier, 54 denotes a limiter amplifier, 55 denotes a detector circuit, 56 denotes a binarization circuit, 57 denotes a microcomputer, 58 denotes a light emitting diode, and 59 denotes a driving transistor.
Further, there is disposed a conveyer belt (not shown) between the photodiode 50 and the light emitting diode 58, and, on the conveyer belt, the transported objects are transported with a suitable distance therebetween.
In the above described arrangement, a timing circuit (not shown) incorporated in the microcomputer 57 generates a driving pulse with a rectangular waveform at the timing of the transported object passing close by one of the light emitting diodes 58. The driving pulse is supplied to the light emitting diode 58 through the light emitting diode driving circuit, so that the light emitting diode 58 emits light every time the transported object passes close by it and throws the optical output power on the transported object. The reflected light from the transported object is detected by the photodiode 50 and a detected signal corresponding to the intensity of the reflected light is output therefrom. Then, the detected signal is amplified to a predetermined level by the preamplifier 51, deprived of unnecessary components by the low-pass filter 52, and amplified at high gain by the amplifier 53. Then, the amplified detected signal is subjected to limiting and amplifying processing in the limiter amplifier 54 such that one of the polarities is chiefly amplified, e.g., a positive half-wave component is amplified, and then subjected to detection in the detector circuit 55 such that the envelope of the signal component is detected. The envelope signal is converted into a binary signal by the binarization circuit 56 and supplied to the microcomputer 57. The microcomputer 57 makes decision as to presence or absence of the transported object on the basis of the binary signal.
In such a light emitting diode driving circuit for use in a photoelectric switch apparatus or the like, the optical output power of the light emitting diode 58 tends to gradually attenuate by the effect of changes in the ambient temperature, age deterioration, and the like, even if there is no change in the power of the driving pulse supplied to the light emitting diode 58. The attenuation of the optical output power causes attenuation of the reflected light from the transported object accompanied by lowering of the signal level of the detected signal from the photodiode 50. When the signal level of the detected signal lowers, there arises a problem that the processing of the detected signal becomes difficult, especially the decision as to presence or absence of the transported object according to the binary signal obtained by the binarization process becomes substantially difficult, and consequently it occurs that erroneous decision is made as to presence or absence of the transported object.